As a high energy density battery, a non-aqueous electrolyte battery such as a lithium-ion secondary battery is under an extensive research and development in recent years. The non-aqueous electrolyte battery is expected to be a power source for a hybrid vehicle or an electric vehicle or an un-interruptible power source for a base station of mobile-phones. However, even if a cell of a lithium-ion secondary battery is made large, a voltage obtained from the cell is as low as about 3.7 V. For this reason, to obtain a high output, the cell must be made to be a large-scale to generate a large current. As a result of this, the whole of a device which uses the large-scaled cell becomes bulky.
As a battery to solve these problems, a bipolar battery has been proposed. The bipolar battery is a battery in which a plurality of bipolar electrodes, each having a current collector, a positive-pole active material layer formed on one side surface of the current collector and a negative-pole active material layer formed on the other side surface thereof, are laminated in series with electrolyte layers being interposed between them. Even if such a bipolar battery as described above is used as a cell, a high voltage can be obtained from the cell because the bipolar electrodes are laminated in series in the cell of the bipolar battery. Hence, a high output coming from a high voltage and constant current can be obtained from the bipolar battery. Further, since a high output can be obtained from a small number of the bipolar batteries, the number of the batteries which are needed to obtain the high output are decreased. Therefore, the number of battery connections can be largely decreased and an electrical resistance generated in the battery connections can be largely reduced.
The lithium-ion secondary battery employs a structure using a liquid electrolyte. However, in the cell of the bipolar battery, the positive-pole active material layer and the negative-pole active material layer are arranged repeatedly. Hence, the structure of the lithium-ion secondary battery using the liquid electrolyte is not able to be adapted simply to the bipolar battery. That is, the bipolar battery must be structured to independent and not to contact the liquid electrolytes between electrode layers from each other, so that a short circuit (liquid junction) by ionic conduction will not occur between them due to a contact between the liquid electrolytes that exist between the electrode layers.
A bipolar battery which uses a polymer solid electrolyte not containing a liquid electrolyte has been proposed. In this bipolar battery, a possibility of the short circuit (liquid junction) caused by the ionic conduction between the electrode layers becomes low. However, an ion conductance of the solid electrolyte is generally much lower than that of the liquid electrolyte and is about 1/10 to 1/100 of that of the liquid electrolyte. For this reason, the bipolar battery using the polymer solid electrolyte is not actually used.
Considering these circumstances, a bipolar battery using a gelled electrolyte that is a semi-solidified liquid electrolyte has been proposed. The gelled electrolyte is formed by impregnating a polymer such as polyethylene oxide (PEO) or polyvinylidene fluoride (PVdF) with a liquid electrolyte. The gelled electrolyte has a high ion conductance and is expected to make the bipolar battery generate a sufficient output density.
There is a problem to make a bipolar battery being large (have a high energy density). As a method of making the bipolar battery have a high energy density, for example, increasing area of each of positive-pole and negative-pole of the bipolar battery or connecting cells of bipolar batteries each having a small area in parallel. In a lithium-ion secondary battery having a conventional electrode structure, by tightly and spirally winding the electrodes of positive and negative poles together with separators and by installing them in a battery case at a high density, the conventional lithium-ion secondary battery is made to obtain a high energy density. In the bipolar battery, however, the positive-pole and the negative-pole are integrally formed. Hence, the opposite poles come into contact with each other by the spirally winding. For this reason, a short circuit occurs by the spirally winding, unless a separator or an insulating layer such as a polymer is placed between the bipolar electrode layers.
In this case, however, a thickness of an electrode body being inserted with the separator or the polymer increases, and an insulating efficiency of the electrode lowers. Therefore, it is difficult to make an energy density of the bipolar battery being high by the spirally winding the bipolar electrode in the past. Additionally, in a case in which the electrode area is increased by the spirally winding, it is difficult to set current collection tabs for connecting current collectors of the electrodes to a current terminal of the battery on the current collectors respectively. This means that the larger the area of each electrode is, the higher an internal resistance of the battery is. And, this prevents an output of the bipolar battery from increasing. Hence, a technique for solving these problems of the bipolar battery and for making the bipolar battery to have both a high output/input and a high energy density is required.